DEC-205 is an endocytic receptor expressed by a distinct CD8 alpha+ dendritic cell subpopulation. Targeting of proteins to DEC-205 through chimeric antibody constructs causes clonal deletion or anergy of antigen-specific CD4+ and CD8+ T cells in immunologically normal mice. In NOD mice, a mouse model for autoimmune diabetes, beta cell-specific CD8+ T cells can be depleted by anti-DEC antigen treatment (Mukhopadhaya A, et al. PNAS 2008). We are now assessing the ability of DEC-205 targeting to deplete or anergize CD4+ T cells in autoimmune NOD mice. NOD mice were injected with BDC2.5 T cell receptor (TCR) transgenic CD4+ T cells that recognize an unknown beta cell antigen, followed by treatment with anti-DEC-205 attached to a BDC mimeotope peptide. Three days after anti-DEC-205 treatment, BDC T cells had proliferated in lymphoid organs, similar to results in normal mice. However, ten days after treatment with anti-DEC-205, when T cells are deleted in normal mice, antigen-specific cells remained in the autoimmune mice and were not anergic (they retained capacity to produce interferon gamma). We are now assessing what immune pathways may be important for inducing tolerance in this setting. CD11b+ dendritic cells express DCIR2 on their surface, and antibodies specific for DCIR2 can be used to target antigens to this DC subset. We are now measuring BDC2.5 TCR transgenic T cell responses after stimulation in vivo with anti-DCIR2-targeted BDC peptide in NOD mice. We are also testing whether this antibody-antigen combination can alter diabetes development. Another type of DC, Plasmacytoid DCs (pDC) have traditionally been identified as a distinct subset of DCs involved as an innate immune cell in viral immunity due to their ability to secrete large amount of type 1 IFN upon stimulation. However, more recently, pDCs have also been shown to present antigen to CD4 T cells and to be vital in the induction of immune tolerance. We are now addressing the role of pDC in the pathogenesis of autoimmune diabetes by 1) studying the number of pDCs at different stages of diabetes progression in the NOD mice, and in non autoimmune strains. 2) understanding how diabetes susceptibility genes affect pDC development, focusing on genes found in the susceptibility loci Idd3 and Idd5. In the future, we will deplete pDCs in NOD mice and determine if this affects diabetes development. We have also assessed how IL-2, an autoimmune diabetes susceptibility gene in both mouse and human, affects DC development. Using cultures of bone marrow cells stimulated with FMS-like tyrosine kinase 3 ligand (Flt3L), we have shown that IL-2 inhibits both cDC and pDC development. Furthermore, DCs that do develop in the presence of IL-2 display reduced ability to stimulate T cell proliferation. IL-2 is likely acting at the MDP stage because those are the precursors that express the IL-2Ra, and MDPs accumulate in cultures with Flt3L and IL-2.